1. Field of the Invention
The present invention relates to a contour emphasis circuit and more particularly to a technique used in picking up an image of a microscope by a television camera to optimally emphasize a contour in accordance with a change in a magnification factor of an objective lens.
2. Related Background Art
When an image of an optical equipment such as a microscope is to be picked up by a television camera (TV camera), a contour emphasis circuit is usually used to sharpen an image. FIG. 3 shows a configuration of a prior art contour emphasis circuit. In the circuit of FIG. 3, a video signal from a TV camera for picking up an image of a microscope (not shown) is applied to a video signal input terminal 1, and it is supplied to an adder 4 through a video signal processing circuit 2 which carries out white balance adjustment, .gamma. correction and other processing. On the other hand, the video signal from the video signal input terminal 1 is also applied to a contour extractor 3 which extracts a high frequency component in a predetermined frequency range, that is, a contour component, which is supplied to the adder 4, which in turn adds it to the signal from the video signal processing circuit 2 to produce a sum output from a video signal output terminal 5.
In the prior art contour emphasis circuit, there is no adaptive coupling between the optical equipment such as the microscope and the TV camera, and the contour signal component extracted by the contour extractor which has a constant band regardless of the change of a magnification of an objective lens of the optical equipment is added to the video signal. As a result, exact contour emphasis in accordance with the magnification factor of the objective lens is not attained, and in some cases a signal to noise ratio is deteriorated.
This is more fully explained with reference to FIG. 4. A spatial frequency included in an optical image of the microscope usually varies with the magnification factor of the objective lens. That is, a frequency contained in the contour of the image changes. For example, as shown in FIG. 4(a), spatial frequencies transmissive as an image when the magnification factors of the objective lens is 2, 20 and 100 are shown in the abscissa and output responses therefor are shown in ordinate. It is seen that as the magnification factor increases, the transmissive spatial frequency band is narrowed toward the low frequency. Accordingly, as shown in FIG. 4(b), when the contour components extracted by the contour extractor having the constant band characteristic are added to the spatial frequency components of the respective magnification factors, the extract contour emphasis is not carried out, that is, the high frequency components are not fully emphasized, as shown in FIGS. 4(c) and 4(d). Further, the frequency components which do not include the signal components of the image are amplified so that the contour signal cannot be fully extracted and the noise components shown by hatching are amplified. This lowers an S/N ratio.